Certain agricultural machines, such as balers and forage harvesters, are required to pick up crop material that has been cut but still lies on the ground. The mechanism used to gather the crop material from the ground and feed it into the machine for processing is known as, and herein referred to as, a pickup. A pickup typically comprises a drum rotatably mounted on a frame and having radially projecting tines to drive the crop material upwards, past a guide (also known as a windguard) and then into a feed channel through which the crop material is conveyed into the machine for processing. The construction of the pickup is not of importance to the present invention and has been described only by way of general background.
It is known for a pickup to be supported on two ground wheels and these may be adjustable in order to allow the ground clearance to be adjusted. As the ground wheels are provided only for support, and are not used for steering, they are commonly designed as caster wheels so that they align themselves automatically with the direction of travel of the machine. As is known, a caster wheel is pivotable about a generally vertical axis, generally known as and herein referred to as a caster axis. Typically, the point of contact of the wheel with the ground is offset from the caster axis. As a result, drag rotates the wheel such that its point of contact with the ground always lines up behind the caster axis in the direction of travel thereby causing the wheel to point automatically in the correct direction.
A problem arises with this wheel design when the pickup is transported on public roads with the wheels raised off the ground. When they are not in contact with the ground, the caster wheels can rotate freely and can move to a position where they project laterally beyond the sides of the pickup. In such a position, they may increase the width of the pickup beyond legally permissible limits for transportation on a public highway.
EP 1 362 506 discloses a crop pickup arrangement and height gauge arrangement, in which on the one hand, a skid shoe and on the other hand, a wheel are attached to a pivot arm assembly. The wheel is supported by the assembly for rotation about a horizontal axis and the assembly is itself mounted on the pickup to pivot about a second horizontal axis. This arrangement, which acts in the same way as a motorcycle swinging arm suspension, allows height adjustment of the wheel but the wheel has no caster action and does not align automatically with the direction of travel.
EP 2 113 396 discloses a pickup support wheel mounted on a carrier secured to a pivot shaft that defines the caster axis of the wheel. The pivot shaft can rise and fall relative to the frame of the pickup and, when it falls to the limit of its travel, a pin projecting from the pivot shaft interacts with a cam surface to cause the wheel to rotate into a desired position. However, during normal operation of the pickup there is no interaction between the pin and the cam.
WO 2012/072436 discloses a pickup for an agricultural machine having a frame carrying a pickup mechanism and supported on the ground by means of two support wheels, wherein each support wheel is mounted for rotation about a generally horizontal axis on a carrier that is connected to the frame of the pickup for rotation about a generally vertical caster axis, the projection of the caster axis intersecting the ground at a point offset from the point of contact of the wheel with the ground. Each carrier is connected by a respective spring to a point fixed relative to the frame of the pickup, the spring biasing the carrier to cause the support wheel to be oriented in a predetermined direction relative to the frame of the pickup. In the presented embodiment, the spring is mounted below an arm of the frame of the pickup. When the support wheels are raised from the ground the springs acting on their carriers bias them into their predetermined direction, also known as the parked position, in which they do not project impermissibly beyond the frame of the pickup. However, the spring is attached such that it limits the range of rotation of the wheel. In particular, the wheel is not able to rotate substantially more than 180 degrees from its initial position, which may cause problems when manoeuvring backwards on an uneven terrain, when the wheel is biased to a position rotated 180 degrees with respect to the bias position when manoeuvring forwards.
Therefore, there is a need for an improved caster wheel assembly of a pickup for an agricultural machine which will bias the wheel to a predetermined direction, but provide greater freedom of rotation of the wheel while manoeuvring forwards and backwards.